The present invention relates to a method and apparatus for cooling hot glass fibers or filaments as they are drawn from molten glass in a furnace.
In processes such as the manufacture of glass optical waveguides, glass fibers are drawn at very high speeds from a molten glass preform positioned in a drawing furnace. Very rapid cooling of the glass fiber is required during such manufacture in order to reduce the temperature of the glass to a level at which the fiber may be protectively coated. The speed of cooling can in fact become a critical rate-limiting step for optical fiber production, since the technology of rapidly coating optical fibers, as well as the technology of rapid fiber drawing, is well developed.
A number of different methods for cooling optical fibers at rapid rates have been proposed. U.S. Pat. Nos. 4,437,870 and 4,514,205 utilize cold flowing gases to cool the fibers, while U.S. Pat. No. 4,664,689 utilizes a combination of radiative cooling and cryogenic gas cooling to further increase the cooling rate.
In addition to radiative and gas cooling of optical fibers, it has also been proposed to utilize conductive cooling via the direct application of cooling liquids to the surfaces of the fibers. U.S. Pat. No. 4,583,485 employs a preapplication of liquid coating components to achieve quench-cooling of the fiber surfaces, while U.S. Pat. No. 5,043,001 uses water or a vaporizable organic liquid for fiber cooling. The apparatus used in the latter patent includes a provision for removing the coolant liquid from the surfaces of the fiber prior to coating.
The use of liquid-phase quenching media to cool optical fiber is attractive because of the relatively high heat extraction rates available through the vaporization of cooling liquids. However, the disadvantages of such cooling include a substantial risk, particularly at higher fiber surface temperatures, that surface contact with substantial masses of liquid applied at temperatures much lower than fiber surface temperatures could introduce thermal stresses or cause breakage of the optical fiber. Moreover, as noted in U.S. Pat. No. 5,043,001, for most cooling liquids it is important that complete liquid removal from the surfaces of the cooled fiber be achieved prior to the time fiber coating material is applied to the fiber. The temperature range through which optical fiber must be cooled in order to prepare it for coating is a range extending from about 1500.degree. C. or above down to approximately 50.degree. C.. At temperatures near the upper extreme of this range, radiative cooling is efficient and rapid cooling of the fiber can readily be achieved.
In the mid-range of temperatures, e.g., from about 1000.degree. to about 500.degree. C., radiative cooling becomes less efficient. Therefore, in this range, convective or conductive cooling of the fiber by gases of high thermal conductivity, typically supplied at cryogenic temperatures, is preferred to achieve high cooling efficiencies.
An important shortcoming of present cooling procedures, however, is that of achieving rapid cooling through the lowest temperature regime, i.e., through the temperature range of about 500.degree.-50.degree. C. In this range, neither forced gas convection cooling nor radiative cooling provides rapid enough energy transfer to achieve an efficient reduction of fiber temperatures. This is due to the fact that the differences in temperature between the glass fiber and the cooling medium are relatively small, such that heat transfer rates between the fiber and the cooling medium are low.
For these reasons it is evident that major improvements in cooling efficiency over the lower ranges of fiber surface temperature will be required if further large increases in fiber drawing rates are to be achieved.
It is therefore a principal object of the present invention to provide improved fiber cooling methods, and apparatus for practicing those methods, which offer accelerated fiber cooling particularly at the lower extremes of the conventional fiber cooling range.
Other objects and advantages of the invention will become apparent from the following description.